1. Field of the Invention
The present invention relates to an image display apparatus and, more particularly, to a head- or face-mounted image display apparatus which is fitted on a user's head or face to enable the user to enjoy viewing an image.
2. Related Art
In recent years, head- or face-mounted image display apparatuses have been developed for the purpose of enabling the user to enjoy a wide-screen image personally.
These image display apparatuses have been demanded to be small in both size and weight and superior in portability or mountability, that is, capable of being favorably fitted on the user's head or face. Accordingly, an ocular optical system for projecting and leading an image to an observer's eyeball has also been demanded to be reduced in both size and weight.
In general, however, an ocular optical system produces a distortion of the virtual image. Particularly, in a case where an optical element that has a power is decentered in the ocular optical system, or the optical system has a rotationally asymmetric optical surface, e.g. an anamorphic surface or a toric surface, a distortion which is asymmetric with respect to the center of the image field occurs in general.
For example, let us consider an optical system according to Example 1 of Japanese Patent Application Unexamined Publication (KOKAI) No. 7-104209. As shown in FIG. 20, assuming that .THETA..sub.1 is an angle made between a visual axis 41 and a light ray emitted from an arbitrary point 44 on an image display surface 8 and passing through the center of an observer's pupil 40 via an ocular optical system 42, and .THETA..sub.2 is an angle made between the visual axis 41 and a light ray emitted from a point 45, which is in point symmetry with the point 44 about the center 43 of the image display surface 8, and passing through the center of the observer's pupil 40, the value obtained from the angles .THETA..sub.1 and .THETA..sub.2, i.e. EQU .vertline.tan .THETA..sub.1 -tan .THETA..sub.2 .vertline./tan .THETA..sub.1
is 0.13 at the maximum. Thus, an extremely large asymmetric distortion is produced.
It is uncertain where the observer's pupil is placed in an exit pupil in which an image is observable. The observer's pupil does not always lie in the center of the exit pupil of the ocular optical system. If the observer's pupil is not placed in the center of the exit pupil, even if an asymmetric distortion is not produced by the ocular optical system, an asymmetric distortion may be observed because the observer's pupil is decentered with respect to the ocular optical system.
Particularly, in a case where images are displayed for both of observer's eyes, respectively, so that the observer can view a composite image with both his/her eyes, if the distance between the centers of the exit pupils of the left and right optical systems of the image display apparatus, that is, the interpupillary distance, is not coincident with the interpupillary distance of the observer, the centers of the exit pupils of the ocular optical systems and the observer's pupils are displaced relative to each other. In such a case, an asymmetric distortion is likely to occur.
If a large distortion, particularly a large asymmetric distortion, occurs, it is likely that the observer will be unable to view the correct image leading to a strong sense of incongruity.
There has also been developed an image display apparatus which provides a stereoscopic image by projecting images with a disparity therebetween into an observer's left and right eyes, respectively. In this case, however, if a distortion is produced by the ocular optical system, the disparity is likely to become different from the intended one owing to the distortion, causing the three-dimensional effect to be spoiled. Particularly, in a case where each ocular optical system produces a distortion which is asymmetric with respect to the center of the image field, and there is a difference in the distortion between the images projected into the observer's left and right eyes, the disparity is likely to deviate, causing the image to appear raised or depressed undesirably.
For example, even if, as shown in FIG. 23(a), two identical images are displayed on left and right image display devices, respectively, with the intention of making the observer perceive a plane surface, if the left and right optical systems produce distortions, each of which is asymmetric with respect to the center of the image field, and the images appear as shown for example in FIG. 23(b) to the observer, the observer perceives a concave surface rather than a plane surface.
There has heretofore been known neither an ocular optical system for an image display apparatus which is corrected for the above-described distortions, particularly a distortion which is asymmetric with respect to the center of the image field, nor an ocular optical system for an image display apparatus which is corrected for distortion that is asymmetric with respect to the center of the image field, which occurs when the observer's pupil is not placed in the center of the exit pupil of the ocular optical system.